Typically, the layer structure required for high performance Schottky diodes is significantly different from the layer structure conventionally used for High Electron Mobility Transistors (HEMTs). In the previous work on monolithic integration of HEMTs and Schottky diodes, the diode epi-layers consisting of a lightly-doped Schottky barrier layer and a highly-doped n+ contact layer were grown on the HEMT epi-layers consisting of a high mobility channel with 2 dimensional electron gas (2DEG). See J. Ho et al., GaAs IC Symposium Proceedings, Proceedings, p. 301, 1988. Since the diode structure was stacked on the HEMT structure according to Ho, Ho's fabrication process consists of two separate steps. The first step is to fabricate Schottky diodes and remove the diode epi layers from areas where HEMTs are to be fabricated. The Schottky diodes are typically a vertical structure, where an air-bridge interconnect technology is needed to minimize parasitic capacitances. The second step is to fabricate the HEMTs. This two step process is complicated and thus increases cost of the epitaxial wafers being produced by this fabrication process.
Monolithic integration of high-frequency GaN-HEMTs and GaN-Schottky diodes as disclosed herein is significant because it allows for the design of millimeter-wave and sub-millimeter-wave receiver front-ends which may include low noise amplifiers, diode mixers, low-noise IF amplifiers, and varactor controlled HEMT VCOs all on the same chip. This patent describes device structures and a fabrication technique of monolithically integrated GaN-based HEMTs and Schottky diodes fabricated on a single epitaxial structure. The integrated HEMTs/Schottky diodes are realized using an epitaxial structure and a fabrication process which should reduce fabrication costs compared to prior art techniques. Since the disclosed process preferably uses self-aligned technology, both devices show extremely high-frequency performance by minimizing device parasitic resistances and capacitances. Furthermore, since the Schottky contact of diodes is formed by making a direct contact of an anode metal to the 2DEG channel the resulting structure minimizes an intrinsic junction capacitance due to the very thin contact area size. The low resistance of high-mobility 2DEG channel and a low contact resistance realized by n+GaN ohmic regrowth reduce a series resistance of diodes as well as an access resistance of HEMTs.